- Better integration
- Hydronic heating
- Radiators
Enhancing efficiency: the importance of pipe sizes and flow velocity when switching to low temperature heating
Prerequisites for low temperature heating
Existing HVAC systems are generally renovated with a view to saving energy. However, this savings potential can only be maximised if the building and the system technology are considered as a whole. So before a low temperature heating system can be considered, the building needs to be brought up to par with modern insulation standards. Only then will you be able to maintain an even room temperature and create a comfortable indoor climate with lower system temperatures.
Additionally, you’ll need a suitable heat source such as a condensing boiler or a heat pump as well as heat emitters that are able to transmit the required amount of heat to the room with low flow temperatures. However, before deciding on either of those, it’s important to recalculate the heat demand after insulation and, based on that, the required volume flow velocity.
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Volume flow velocity and delta t
Whereas a regular heating system with panel radiators generally operates with a delta t of 15 or 20 Kelvin, in a heat pump system the temperature difference between flow and return drops to 10 Kelvin for panel radiators or fan-assisted radiators such as Ulow-E, and 5 or 7 Kelvin for fan convectors such as our iVector S2. This lower delta t allows for a high COP of the heat pump, but it increases the water volume flowing through the pipes significantly. If, for example, you need 1200 W heat output for a room, the mass flow rate in case of a gas or oil boiler is 51,6 kg/h at a delta t of 20 Kelvin and 71,5 kg/h at 15 Kelvin.
A heat pump system, however, requires a significantly higher mass flow. If you pair a heat pump with low temperature panel radiators or fan-assisted radiators and use a delta t of 10 Kelvin, the required mass flow increases to 103,3 kg/h. If you would like to combine the heat pump with fan coils at system temperatures of 40/33/20°c, or even 35/28/20°C in combination with underfloor heating, the mass flow rate rises to 147,5 kg/h. Yet, the best COP for the heat pump is achieved at a delta t of 5 Kelvin. This can be realised with fan coils and system temperatures of 40/35/20°C, or 35/30/20°C in combination with underfloor heating. In this case the required mass flow rate further increases to 206,5 kg/h, which is a multiple of the mass flow rate in a standard system.
Read more about the importance of delta t in hydronic heating systems
“That is why it’s vital for planners or installers working on a renovation project to examine the existing pipe network at this stage”, says Heiko Hanke, Product Manager Radiators at Purmo. “It’s important to ensure the pipes are large enough for the increased water volume. Undersized pipes could result in flow restrictions as well as increased pressure losses, preventing the room from getting warm enough because there isn’t enough water flowing through the radiator. Moreover, if the pipes are not properly sized, this often causes noises in the heating system.”
The importance of pipe sizes
We are aware that it isn’t always easy in renovations to get an overview of the entire pipe network as most pipes are hidden in the ceiling or walls. You can, however, always check the connection pipes to the radiators and the main pipe in the boiler room. Once you know the pipe sizes, you can assess whether it’s feasible to switch to a low temperature heating system. When doing so, don’t forget to take into account the standard values for flow velocity, which are 0.3 to 1.5 m/s in the main distribution pipes and 0.5 to 0.8 m/s in the radiator connection pipes. Average pressure drops in the system are 50 to 100 Pa/m in a standard family house and up to 200 Pa/m for larger systems.
Family houses built after the 1970s usually have copper connection pipes of 12x1 mm with 10 mm inner diameter or 15x1 mm with 13 mm inner diameter. Multilayered heating pipes are also possible in more recent buildings. Although these are thicker, they have the same inner diameter, which is what matters for the water flow. Taking into account a flow velocity of 0.5 m/s, the maximum water volume is 141 kg/h for the 10 mm pipes. For 15x1 mm connection pipes with 13 mm inner diameter this increases to 238 kg/h, allowing you to lower the system temperatures, and more importantly, delta t.
Aligning all system components
The calculations mentioned above provide important insights to ensure that a renovated heating system functions optimally. Heiko explains: “Once you know the volume flows that are possible within the current pipe network, you can compare this with the required volume flows and delta t for an optimally functioning heat source and emitters in a low temperature system. If there is a match and the pipes allow for higher water volumes, you can easily install a condensing boiler or heat pump and replace older panel radiators by low temperature emitters such as fan-assisted radiators or fan convectors of roughly the same size for maximum energy efficiency. In most cases this is feasible, especially when the heat demand of the building is reduced. Sometimes, however, the pipes are undersized and then you have to decide between installing a new pipe network, which is often not feasible in existing buildings, or using a higher delta t.”
“Although a higher delta t might not allow the new heat source to operate as efficiently as possible, it’s important to find a compromise that allows for an optimally functioning system with fully aligned components. In practice, the heat source and/or emitters are often still replaced blindly without checking the pipes, but then the system simply cannot function properly.”
At Purmo we strongly believe that a better integration of all components brings multiple performance optimisation benefits. We are therefore happy to assist our customers in creating the greatest possible indoor climate comfort. So, if you have any questions about pipe sizes and flow velocity for a low temperature heating system, don’t hesitate to reach out to our experts for tailored advice.